Separation process for carpets

ABSTRACT

Comminuted carpet pieces are fed to a first stirred tank together with an aqueous solution containing separating salt, to form a first suspension, which is fed to a first mechanical separating stage. A first high-solids phase, a second high-solids phase containing polymer fiber material, and a liquid phase are obtained therefrom. The second high-solids phase is mixed with a water-containing separation solution in a stirred tank, to give a second suspension, which is fed to a second mechanical separating stage. A third high-solids phase, a polymer fiber material-rich phase and a liquid phase are withdrawn therefrom. An acid which is stronger than H 2 CO 3  is introduced into the stirred tank together with the water-containing separation solution, and the pH of the liquid in the stirred tank is adjusted to 2–6.

This is a 371 of PCT/EP02/00159 filed 10 Jan. 2002 (international filingdate).

The invention relates to a process for separating polymer fiber materialfrom comminuted carpet pieces, which are fed to a first stirred tanktogether with an aqueous solution containing separating salt, and afirst suspension is withdrawn from the first stirred tank and fed to afirst mechanical separating stage, from which a first high-solids phase,a second high-solids phase containing polymer fiber material, and aliquid phase are separated off, the second high-solids phase being mixedwith a water-containing separation solution in a second stirred tank,and a second suspension being withdrawn from the second stirred tank andfed to a second mechanical separating stage, from which a thirdhigh-solids phase, a polymer fiber material-rich phase, and a liquidphase are separated off.

BACKGROUND OF THE INVENTION

A process of this type is disclosed in EP 0681 896 B1. In this process,the carpet is wet-ground in two stages and separated into threefractions in two centrifuge separating stages connected in series.

SUMMARY OF THE INVENTION

The invention is based on the object of improving the known process andobtaining polymer fiber material, for example polyamide fibers, in thehighest possible purity in an inexpensive manner. This is achieved inaccordance with the invention in the process mentioned at the outset inthat an acid which is stronger than H₂CO₃ is introduced into the secondstirred tank together with the water-containing separation solution, andthe pH of the liquid in the second stirred tank is adjusted to 2–6. Dueto the acidification in the second stirred tank, residues of chalk aredissolved and can be separated off completely. Chalk-containingparticles, in particular rubber particles, lower their specific gravitydue to the dissolution of the chalk and can be discharged with the thirdhigh-solids phase through the mechanical separating stage. Suitableacids which are stronger than H₂CO₃ are, for example, nitric acid,hydrochloric acid, acetic acid or formic acid.

DETAILED DESCRIPTION

The separating salt fed to the first stirred tank is advantageously acalcium salt which is readily soluble in water and whose acid dissolvescalcium carbonate. Suitable calcium salts are, for example, calciumnitrate and calcium chloride. However, it is alternatively also possibleto use a readily soluble salt such as, for example, K₂CO₃.

One or both separating stages preferably operate with a double-conefull-jacket centrifuge. Before each centrifuge, a suspension whosesolids concentration is in the range of 1–15% by weight and preferably3–10% by weight is produced in the associated stirred tank. It isensured that the density of the liquid phase of the suspension in thefirst separating stage is set to a value which is between the highestand second-highest density of the principal components of the carpet. Inthe second separating stage, the density of the liquid phase is betweenthe lowest and the second-lowest density of the principal components ofthe carpet. The carpet component with the highest specific gravity isnormally, due to the filler, the carpet backing material, with a densityof about 1.5–3.5 kg per liter. The second-heaviest component is formedby the pile fibers, with a density of about 1.35 kg per liter forpolyester and about 1.15 kg per liter for polyamide. Correspondingly,for example, the density of the liquid phase of the suspension in thefirst separating stage in the case of recovery of polyester fibers isset to a value in the range from about 1.36 to about 1.5 kg per literand in the case of polyamides in the range from 1.16 to 1.3 kg per liter(at 20° C.). The separating salt used here plays an important role insetting the density of the first suspension. In the second stirred tank,the setting of the pH in the range 2–6 by addition of acid is also ofimportance in addition to the density. The addition of small amounts ofless than 0.2 g per liter of a commercially available wetting agentand/or antifoam agent to the first and/or second stirred tank isadvisable.

Particularly good separation results are achieved with the double-conefull-jacket screw centrifuge. With this commercially availableapparatus, it is possible to separate the fraction of the solid having ahigher density than the liquid phase from the fraction having a lowerdensity than the liquid phase and to obtain a liquid phase as the thirdphase. The two solid phases are discharged with a residual moisturecontent in the range from 2 to 25% by weight, depending on theconsistency of the particles. The heaviest solid fraction obtained inthe first separating stage usually consists principally of mineralfillers, for example synthetic rubber to which chalk has been added, andcontains only small amounts of fiber material. After drying, thisfraction can be passed to thermal energy recovery or material recycling.

A refinement of the invention consists in that the polymer fibermaterial of the comminuted carpet pieces consists at least partly ofpolyamide fiber material, and in that the second high-solids phase,calculated in dry form, consists of at least 50% by weight of polyamidefiber material. A polymer fiber material-rich phase which consists of atleast 90% by weight of polyamide fiber material is preferably withdrawnfrom the second separating stage. This polyamide fiber material can bere-used without difficulties, since it has the requisite purity for use,for example, as raw material in recycling methods.

Embodiments of the process are explained with the aid of the drawing,which shows a flow chart of the process.

BRIEF DESCRIPTION OF THE DRAWINGS

The carpet waste to be processed is fed, on transport route (1), firstlyto a precomminution step (2), for example a shredder. The pre-comminutedmaterial then passes either on transport route (2 a) to a hammer mill(3) or on transport route (2 b) directly to fine comminution in acutting mill (5). The carpet material passing through the hammer mill(3), which is usually soiled to a relatively great extent, is passedthrough line (6) to a sieve (7), which has mesh widths approximately inthe range of 3–10 mm. The dirt adhering to the carpet is separated offtogether with a fraction of chalk and removed through line (8), and thecarpet pieces pass on route (7 a) to the cutting mill (5). The route viathe hammer mill (3) and the sieve (7) has the advantage thatwear-promoting dirt, for example sand, is removed before entry into thefine comminution step. Furthermore, the material flow coming from theshredder (2) is reduced. This enables subsequent plant parts to be builtsmaller, and subsequent losses of salt and water which occur via theresidual moisture content of the solids are reduced.

The discharge from the cutting mill takes place via a perforated platewith holes approximately in the range of about 3–10 mm, preferably 4–8mm. Due to the comminution, substantially complete separation of thecarpet composite takes place. The comminution can be carried out eitherin dry or wet form, depending on the degree of soiling of the rawmaterial. In the latter case, water is supplied through line (5 a) andsoiled water is discharged through line (5 b).

Comminuted carpet material having piece sizes approximately in the rangeof 3–20 mm is passed through line (9) to a first stirred tank (10), towhich a first aqueous solution containing separating salt is also addedthrough line (11). A first liquid phase which is fed back is likewiseintroduced into tank (10) through line (12). A first suspension isproduced, which is fed to a first separating stage (14) through line(13). The first suspension has a solids content of usually 1–15% byweight, and the density of the liquid phase in the case of recovery ofpolyamide fiber material is usually in the range of 1.16–1.2 kg perliter.

From the first separating stage (14), a first high-solids phase, whichis the heavy phase containing rubber and chalk, is withdrawn throughline (16). The second high-solids phase, which contains the polymerfiber material to be recovered, is withdrawn in line (17) and introducedinto a second stirred tank (20). The liquid phase likewise formed in thefirst separating stage is fed back in line (12).

An aqueous, acid-containing separation solution is fed to the secondtank (20) through line (21), and a recycled liquid phase coming fromline (22) is also introduced into tank (20). The acid is stronger thanH₂CO₃ in order that it readily dissolves chalk in particular. Asuspension whose liquid has a pH of 2–6, so that, inter alia, all thecalcium compounds dissolve, is produced in tank (20). The suspensionproduced passes through line (24) with a solids content of 1–15% byweight into the second separating stage (25), which, like the firstseparating stage (14), can be a double-cone full-jacket centrifuge. Thedensity of the liquid in the suspension in line (24) is usually 1.0–1.12kg per liter in the case of the recovery of polyamide fiber material.The heavier high-solids phase, which usually consists of pile fibers, iswithdrawn from the second separating stage through line (26). Thismaterial can be fed to a depolymerization plant. The desired, polymerfiber material-rich phase, which may consist, for example, of at least90% by weight of polyamide fibers, is obtained in line (27). The liquidphase likewise separated off is partly fed back in line (22), and acertain proportion thereof can be removed from the process through line(28) and fed to purification (30), for example a thickener. Purifiedaqueous separation solution is fed back in line (31). The soiled waterformed in line (5 b) can also be introduced into the thickener (30).

EXAMPLE 1

Use is made of a plant corresponding to the drawing, to which are fed 50kg of carpet having the following composition through transport route(1):

Pile material comprising polyamide fibers (PA) 35% by weightPolypropylene as backing fabric (PP)  7% by weight Foam backing ofchalk-containing styrene-butadiene 55% by weight latex (SBR) Dirt  3% byweight

The carpet is pre-comminuted in a shredder (2) fitted with a 90 mmsieve. The dirt adhering to the pieces and other fine fraction aresubsequently mechanically loosened by means of a hammer mill (3). Thecarpet is passed via transport route (6) to the sieving machine (7).This separation device is fitted with a sieve having a mesh width of 5mm. 2.6 kg of dirt and other fine fraction are separated off from theunderflow.

The shredded carpet from the top flow of the sieving machine is passedto the cutting mill (5), which is fitted with a perforated plate havingsquare holes with a diameter of 6 mm. The carpet is finely comminuted indry form in this device. The ground carpet is passed via line (9) to thestirred tank (10). The tank contains 900 kg of a separation solution setto a density of 1.17 kg/l by means of Ca(NO₃)₂, some of which has beeninitially introduced via line (12). In order to set the density, thesolution contains 194 kg of Ca(NO₃)₂. The suspension prepared is passedvia line (13) to a double-cone full-jacket screw centrifuge (14).

The first high-solids phase (dry weight: 26.2 kg) is discharged with aresidual moisture content of 22% by weight through line (16) and thesecond high-solids phase (dry weight: 21.1 kg) is discharged with aresidual moisture content of 12% by weight through line (17), and theseparation solution (12) is fed back into the stirred tank (10). Thesecond high-solids phase is introduced into the second stirred tank (20)and suspended in 399 kg of water. 2.4 kg of nitric acid (21) having aconcentration of 50% are added to the suspension prepared in the stirredtank (20), to give a pH of 4.

The suspension prepared is introduced into the second separating stage(25) via line (24). The third high-solids phase (dry 4.7 kg) isdischarged with a residual moisture content of 14% by weight throughline (27), and the fourth high-solids phase (dry 16.4) is dischargedwith a residual moisture content of 16% by weight through line (26). Theseparation solution (22) is partly fed back into the stirred tank (20),and a sub-stream (28) of 40 kg is removed and fed to the thickener (30).After clarification and removal of 0.1 kg of impurities, re-introductioninto the separation circuit in the 2nd stage (line (31)) takes place.

The individual high-solids phases have the following composition, basedon the dry matter:

PA PP SBR Dirt First high-solids phase  3.4% by wt.  0.1% by wt. 94.8%by wt.  1.7% by wt. Second high-solids phase 77.8% by wt. 16.2% by wt. 5.9% by wt.  0.1% by wt. Third high-solids phase  8.7% by wt. 90.3% bywt.  1.0% by wt.  0.0% by wt. Fourth high-solids phase 98.2% by wt. 0.2% by wt.  1.5% by wt.  0.1% by wt. Sieve residue  6.6% by wt.  1.3%by wt. 52.2% by wt. 39.8% by wt.

EXAMPLE 2

Use is likewise made of a plant corresponding to the drawings to which50 kg of carpet having the following composition are fed throughtransport route (1):

Pile material comprising polyamide fibers (PA) 35% by weightPolypropylene as backing fabric (PP)  7% by weight Foam backing ofchalk-containing styrene-butadiene 57% by weight latex (SBR) Dirt  1% byweight

The carpet is pre-comminuted in a shredder (2) as in Example 1. Theshredded carpet is subsequently passed via line (2 b) to the cuttingmill (5). The cutting mill is fitted with a perforated plate havingsquare holes with a diameter of 6 mm. The carpet is finely comminuted inthis device with addition of 200 kg of water. 150 kg of water and 1.0 kgof dirt are discharged from the mill and fed via line (5 b) to thethickener (30). 50 kg of water remaining in the raw material are fed tothe stirred tank (10) together with the ground carpet via line (9). Aseparation solution prepared by means of Ca(NO₃)₂ is initiallyintroduced into the tank via line (12) and is set to the density 1.17kg/l by means of the 50 kg of water introduced from the mill, giving atotal amount of 931 kg of separation solution. In order to set thedensity, the solution contains 201 kg of Ca(NO₃)₂. The suspensionprepared is fed via line (13) to a double-cone full-jacket screwcentrifuge (14). The first high-solids phase (dry weight: 27.8 kg) iswithdrawn with a residual moisture content of 22% by weight through line(16), and the second high-solids phase (dry weight: 21.2 kg) isintroduced with a residual moisture content of 12% by weight throughline (17) into the second stirred tank (20), where it is suspended in400 kg of water. The separation solution from line (12) is fed back intothe stirred tank (10).

2.6 kg of nitric acid having a concentration of 50% are added to thesuspension prepared in the stirred tank (20), and the pH is set to 3.5.The suspension prepared is introduced into the second separating stage(25) via line (24). The third high-solids phase (dry 4.8 kg) isdischarged with a residual moisture content of 14% by weight into line(27), and the fourth high-solids phase (dry 16.4) is withdrawn with aresidual moisture content of 16% by weight in line (26).

The separation solution (22) is fed back into the stirred tank (20). Asub-stream (28) of 40 kg is removed and fed to the thickener (30). Afterclarification and removal of 1.1 kg of impurities, re-introduction intothe separation circuit in the 2nd stage takes place through line (31).To this is added 150 kg of water, introduced into the thickener throughline (5 b).

The individual high-solids phases have the following composition, basedon the dry matter:

PA PP SBR Dirt First high-solids phase  3.2% by wt.  0.1% by wt. 95.9%by wt. 0.7% by wt. Second high-solids phase 77.5% by wt. 16.2% by wt. 6.3% by wt. 0.0% by wt. Third high-solids phase  8.7% by wt. 90.2% bywt.  1.1% by wt. 0.0% by wt. Fourth high-solids phase 98.2% by wt.  0.2%by wt.  1.6% by wt. 0.0% by wt.

1. Process for separating polymer fiber material from comminuted carpetpieces, in which said comminuted carpet pieces are fed to a firststirred tank together with an aqueous solution containing separatingsalt, a first suspension is withdrawn from said first stirred tank andfed to a first mechanical separating stage, a first high-solids phase, asecond high-solids phase containing polymer fiber material, and a liquidphase are separated off in the first mechanical separating stage, theseparated second high-solids phase is mixed with a water-containingseparation solution in a second stirred tank, and a second suspension iswithdrawn from the second stirred tank and fed to a second mechanicalseparating stage, a third high-solids phase, a polymer fibermaterial-rich phase, and a liquid phase are separated; off from thesecond mechanical separating stage, and wherein an acid which isstronger than H₂CO₃ is introduced into the second stirred tank togetherwith the water-containing separation solution, and the pH of the liquidin the second stirred tank is maintained at 2–6.
 2. Process according toclaim 1, wherein the separating salt fed to the first stirred tank is acalcium salt whose acid dissolves calcium carbonate.
 3. Processaccording to claim 1, wherein the polymer fiber material of thecomminuted carpet pieces is comprised at least partly of polyamide fibermaterial, the second high-solids phase, calculated in dry form, iscomprised of at least 50% by weight of polyamide fiber material, and thepolymer fiber material-rich phase coming from the second mechanicalseparating stage is comprised of at least 90% by weight of polyamidefiber material.
 4. Process according to claim 1, wherein the carpetpieces to be processed come from dry comminution or wet comminution. 5.Process according to claim 2, wherein the polymer fiber material of thecomminuted carpet pieces is comprised at least partly of polyamide fibermaterial, the second high-solids phase, calculated in dry form, iscomprised of at least 50% by weight of polyamide fiber material, and thepolymer fiber material-rich phase coming from the second mechanicalseparating stage is comprised of at least 90% by weight of polyamidefiber material.
 6. Process according to claim 2, wherein the carpetpieces to be processed come from dry comminution or wet comminution.